Structural engineers and Auckland Council staff are assessing a retail and apartment building in Newmarket after a main beam was damaged in a fire.
The blaze broke out in the basement of the King Square Apartments on Broadway this afternoon.
Witnesses said they saw a group of young people running from the area around the time of the fire.
About 100 apartments are in the building and residents are unable to return tonight because of water and power outages.
The Newmarket Business Association was trying to find accommodation those who need it.
The rubbish fire sent smoke billowing across Newmarket and saw the train station evacuated and closed for some time.
Witnesses said thick dark smoke started pouring out of a car park near the station, with ten fire appliances being sent to the scene.
Witnesses told RNZ they heard an explosion after seeing a group of young people fleeing the car park.
FENZ said it did not know anything about an explosion.
Assistant area commander Dave Woon said the amount of heat in the underground car park was causing concern.
"There's been a large amount of rubbish that has caught fire and there's been a severe amount of heat that has damaged the ceiling of the basement which has seen it sag and, not collapse, but sag and we're concerned about the integrity."
Woon said suspicious behaviour was a likely reason for the fire but it was too early to confirm.
Quin Hukui was working near the carpark at the time and saw the fire happen.
He said he was confident the young people seen fleeing the scene had something to do with it.
"We just heard a boom and we were wondering what's going on.
"We had a look and there was a fire going on. Next thing you know, you see police, fire engines, and just a whole lot of kids running off."
Hukui said the group was long gone before police arrived.
Newmarket train station was reopened at 4:45pm, AT said.
"Remuera Road is now open in both directions. Newmarket Station is also now open," it said in a tweet.
"Buses are now returning to their normal routes, including bus route 75.
"Delays and possible cancellations are expected while any late running buses return to schedule."
An RNZ reporter at the scene said there was a strong smell of burnt plastic.